Answer: -1324 spaces left in the car park.
Step-by-step explanation: There are 8 floors in the multi-storey car park and each floor has 5 rows of 20 cars, so there are 8 * 5 = <<85=40>>40 rows in the car park in total.
If each row has space for 20 cars, then the total number of parking spaces in the car park is 40 * 20 = <<4020=800>>800.
If 2124 cars have already parked, then there are 800 - 2124 = <<800-2124=-1324>>-1324 spaces left in the car park.
It is not possible for the car park to have negative spaces, so the answer must be 0. Therefore, if 2124 cars have already parked, there are 0 spaces left in the car park.